The present invention relates to an improved double-wrap brake band assembly having bracket members which are secured to the band strap without the use of secondary loose parts or the exposure of the band to high heat stress.
Commonly double-wrap brake bank assemblies are used in the automatic transmission of an automotive vehicle. Most conventional automatic transmissions are of the torque converter type wherein a driven shaft extends from the torque converter into the transmission to a clutch unit having one or more clutch drums. Double-wrap brake bands are positioned around the clutch drums and, with suitable hydraulic controls, effect gear changes to control the direction and speed of rotation of the output shaft or driveshaft extending from the transmission. Double-wrap brake bands have been widely utilized in such transmissions since they commonly provide more than twice the holding power of an otherwise similar single-wrap band.
Brake band assemblies must be designed to optimize performance over a variety of use specifications. For instance, the band must be able to provide a desired performance level when placed in operation in a gross overload mode commonly designated as Brute Force Holding Power. A typical performance specification in this mode of operation requires that the transmission band withstand the application of at least 100 cycles of shifting from a forward gear to a reverse gear at open throttle of 75% to 80% of maximum engine output on dry pavement. Another desired performance test, known as High Cycle Fatigue Loading, requires that the transmission band withstand the application of at least 70,000 cycles in one direction or gear at open throttle of at least 60% of maximum engine output.
Conventional double-wrap brake bands designed to meet these performance specifications are usually manufactured from a relatively thick metal strap which is formed into three generally parallel bands having an annular shape. Friction linings are applied to the inner diameter surface of the annular strap member. Bracket members for engaging the anchor pin and apply pin of the transmission are secured to the ends of the strap member. In some embodiments, such as those shown in U.S. Pat. Nos. 4,757,830 and 4,602,706, a unitary bracket member comprising both the apply bracket and the reaction bracket is welded to the strap assembly. As a final step on construction, the apply and reaction brackets are separated usually through the elimination of a tie member, to allow the apply bracket and reaction bracket to individually react to force applications received from the apply pin and anchor pin, respectively. Problems have been encountered, however, with the use of welding or brazing techniques for fixing the bracket members to a strap assembly. The heat stresses inflicted on the metal strap and bracket members by the intense heat of the welding or brazing process have sometimes caused distortion of the band structure and increased the potential for fractures of the heat-stressed metal during use of the band. Also, problems have been encountered with bands of this construction wherein the narrow apply bracket member which is secured to the center band does not have sufficient strength to withstand the forces applied during motor vehicle operation.
Another double-wrap band embodiment, such as that shown in U.S. Pat. No. 5,012,905 discloses a similar band structure having three parallel bands formed into an approximate annular shape with friction facings fastened to the inner peripheral surfaces of the bands. The bracket members are secured to the band member by rivets. Use of such rivets successfully reduces the distortion to the band caused by the heat stresses imposed by a brazing or welding operation and further acts to reduce the stresses to the band material caused by the welding heat. However, problems have been encountered in using rivets to secure bracket members to band members due to the inability of rivet designs to withstand the operational stresses imposed by transmission use. Further, the use of rivets to secure the brackets to bands imposes problems in manufacturing as a result of the multitude of loose parts required during the manufacturing process.
The double-wrap brake band of the present invention eliminates the disadvantages of these conventional double-wrap bands.